Wire electrical discharge machining apparatus

ABSTRACT

A wire electrical discharge machining apparatus includes a unit capable of separately opening and closing each of a high impedance path and a low impedance path, a unit that sets an open/close pattern in which a combination of closing one of the feeding paths and opening another one of the feeding paths is designated for switching power feeding between the high impedance path and the low impedance path, a unit that changes pulse energy per feeding pulse in a present feeding path to reduce a difference in discharge pulse energy applied to an inter-electrode gap from a machining power supply between at a time of high-impedance-path feeding and at a time of low-impedance-path feeding, and a unit that controls opening and closing of the path open/close unit in accordance with the changed open/close pattern.

TECHNICAL FIELD

The present invention relates to a wire electrical discharge machiningapparatus.

BACKGROUND ART

In a wire electrical discharge machining apparatus, a wire as oneelectrode is running in an up-down direction and is arranged to beopposed to a workpiece as the other electrode that is controlled to moveon a plane perpendicular to the wire running direction. A pulsedischarge is caused in an opposing gap between the wire and theworkpiece (i.e., inter-electrode gap), and the workpiece is machinedinto a desired shape by utilizing heat energy generated due to thedischarge.

In the wire electrical discharge machining apparatus, in a configurationfor supplying power to the machining electrodes, the workpiece isdirectly connected to one electrode of a machining power supply and therunning wire is connected to the other electrode of the machining powersupply through a feeding point on which the wire is slidable. Generally,two feeding points are provided; one above and the other below theworkpiece. In other words, there are two circuits in parallel on upperand lower sides of the workpiece as paths for a discharge currentsupplied to the inter-electrode gap.

The wire electrical discharge machining apparatus generally employs twomachining power supplies: a sub discharge power supply for inducingspark discharge (pre-discharge) with small current and a main dischargepower supply for supplying large current as a machining current aftergeneration of the spark discharge to perform rough machining and finishmachining.

In the wire electrical discharge machining apparatus, wire breakagesometimes occurs depending upon the machining conditions. The wireelectrode is locally overheated, which results in wire breakage.Conventionally, various technologies have been proposed for preventingwire breakage by preventing the local overheating of the wire electrode(for example, see Patent Documents 1 to 3 (JP 59-47123 A, JP 1-97525 Aand JP 6-61663 B2, respectively) and the like).

Specifically, a technology is disclosed in Patent Document 1 (JP59-47123 A) in which switching elements are provided on each dischargecurrent path from a main discharge power supply to upper and lower sidefeeding points for opening and closing the discharge current pathsindividually so that a one-side feeding for supplying a main machiningcurrent from only one of the feeding points is performed, and anupper-side feeding only from the upper-side and a lower-side feedingonly from the lower side are switched every predetermined number ofcontinuously applied pulse voltages. With this technology, large currentcan be applied without overheating the wire electrode, enabling toprevent wire breakage due to the heat generation.

In Patent Document 2 (JP 1-97525 A), a technology is disclosed in whichswitching elements are provided on each discharge current path from amain discharge power supply to upper and lower side feeding points foropening and closing the discharge current paths individually so that aone-side feeding for supplying a main machining current from only one ofthe feeding points is performed, and an upper-side feeding and alower-side feeding are switched asynchronously. With this technology,occurrence of a concentrated discharge can be prevented, so thatbreakage of the wire electrode due to heating can be prevented.

In Patent Document 3 (JP 6-61663 B2), a technology is disclosed in whicha device is provided for measuring a discharge position in a thicknessdirection of a workpiece based on a difference and a magnitude relationof current flowing from a sub discharge power supply to an upper-sidefeeding point and a lower-side feeding point, and switching elements areprovided on each current path from a main discharge power supply to theupper-side feeding point and the lower-side feeding point for openingand closing the current paths individually. When spark discharge occurson the upper end side in the thickness direction of the workpiece, theupper-side feeding is performed, when spark discharge occurs on thelower end side in the thickness direction of the workpiece, thelower-side feeding is performed, and when spark discharge occurs at thecenter of a workpiece in the thickness direction, anupper-and-lower-both-side feeding for supplying current fromupper-and-lower-both sides simultaneously is performed. The localoverheating of the wire electrode in the center in the up-down directionin the thickness direction of the workpiece in which cooling effecttends to be insufficient can be prevented by switching the feedingsystem in accordance with the discharge position.

In a wire electrical discharge machining apparatus, as disclosed inPatent Document 2 (JP 1-97525 A), machining liquid nozzles are generallyprovided on the wire running path between the upper and lower wireguides at positions that are close in the up-down direction with anopposing position to the workpiece therebetween, and a wire electrode iscooled and discharge machining swarf is removed by ejecting ahigh-pressure machining liquid into the machining gap from upward anddownward.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In the upper-and-lower-both-side feeding system, if there is deviationof impedance between two circuits in parallel on upper and lower sidesof the workpiece as paths for a discharge current, it results in causingdifference between discharge current values supplied from the upper-sidefeeding path and the lower-side feeding path to a discharging position.Therefore, when concentrated discharge occurs, a wire electrode isoverheated on a side having a larger discharge current value, causingwire breakage more easily. Performing the upper-side feeding and thelower-side feeding as disclosed in Patent Documents 1 (JP 59-47123 A)and 2 (JP 1-97525 A) is effective in preventing wire breakage due to theconcentrated discharge; however, when only one-side feeding isperformed, short circuit occurs frequently, thereby lowering machiningspeed.

On the above point, in the technology disclosed in Patent Document 3 (JP6-61663 B2), the one-side feeding system and theupper-and-lower-both-side feeding system are used in combination, andtherefore the technology disclosed in Patent Document 3 (JP 6-61663 B2)is considered to enable stable machining by preventing frequent shortcircuit occurrences. However, there still remains a problem of deviationof impedance that may occur between the upper-side feeding path and thelower-side feeding path.

A discharge current path at the time of upper-and-lower-both-sidefeeding is equivalent to two circuits as discharge current paths inparallel at the time of the one-side feeding. Therefore, impedance ofthe discharge current path at the time of the upper-and-lower-both-sidefeeding is equivalent to ½ impedance of the discharge current path atthe time of the one-side feeding. That is, the power feeding at the timeof the one-side feeding is from a high impedance path, and the powerfeeding at the time of the upper-and-lower-both-side feeding is from alow impedance path. In this case, discharge pulse energy at the time ofthe upper-and-lower-both-side feeding is set not to be excessive inPatent Document 3 (JP 6-61663 B2), so that the discharge pulse energy atthe time of the one-side feeding is made small, creating a problem thatthe intended machinable speed can not be sufficiently utilized withoutoccurrence of wire breakage.

In the wire electrical discharge machining apparatus, a mixed mode ofpower feeding from a high impedance path and power feeding from a lowimpedance path is applied to, for example, one of the upper-side feedingpath and the lower-side feeding path as well as a combination use of theone-side feeding and upper-and-lower-both-side feeding. In such cases,the same problem can be pointed out.

The present invention has been achieved in view of the above discussion,and it is an object of the present invention to provide a wireelectrical discharge machining apparatus enabling improving machiningspeed at power feeding from a high impedance path when the highimpedance path and a low impedance path are used in combination for amain discharge current path.

Means for Solving Problem

To achieve the above objects, there is provided a wire electricaldischarge machining apparatus, when comprising a high impedance path anda low impedance path as feeding paths for supplying a main dischargecurrent from a machining power supply to an inter-electrode gap betweena wire electrode and a workpiece, includes a path open/close unitcapable of separately opening and closing each of the high impedancepath and the low impedance path; an open/close pattern setting unit thatsets an open/close pattern in which a combination of closing one of thefeeding paths and opening another one of the feeding paths is designatedfor switching power feeding between the high impedance path and the lowimpedance path; a feeding-pulse-energy changing unit that generates anew open/close pattern in which a pulse energy per feeding pulse ischanged in a present feeding path indicated by the open/close pattern bythe open/close pattern setting unit such that a difference in dischargepulse energy applied to the inter-electrode from the machining powersupply is reduced between at a time of power feeding from the highimpedance path and at a time of power feeding from the low impedancepath; and a drive unit that controls opening and closing of the pathopen/close unit in accordance with the new open/close pattern generatedby the feeding-pulse-energy changing unit.

According to the present invention, a difference in feeding pulse energybetween at the time of power feeding from a high impedance path and atthe time of power feeding from a low impedance path can be made small,enabling to improve machining speed at power feeding from the highimpedance path.

Effect of the Invention

According to the present invention, when a high impedance path and a lowimpedance path are used in combination for a main discharge currentpath, machining speed at power feeding from a high impedance path can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according toa first embodiment of the present invention.

FIG. 2 is a diagram for explaining a difference in impedance between atthe time of both-side feeding using upper-and-lower-both-side feedingpaths and at the time of one-side feeding using one of feeding paths.

FIG. 3 is a diagram for explaining current waveforms at power feedingsby various feeding systems shown in FIG. 2.

FIG. 4 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according toa second embodiment of the present invention.

FIG. 5 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according toa third embodiment of the present invention.

FIG. 6 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according toa fourth embodiment of the present invention.

FIG. 7 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according toa fifth embodiment of the present invention.

EXPLANATIONS OF LETTERS OR NUMERALS  1 wire electrode  2 upper-side wireguide  3 lower-side wire guide  4 workpiece  5 upper feeding point  6lower feeding point  7 sub-discharge power supply  8 main-dischargepower supply  8a small-current main-discharge power supply  8blarge-current main-discharge power supply  9 upper sub-feeder line 10upper sub-switching element 11 lower sub-feeder line 12 lowersub-switching element 13 upper main-feeder line 13a uppermain-high-impedance-feeder line 13b upper main-low-impedance-feeder line14, 14a, 14b upper main-switching element 15 upper main-feeder line 16lower main-switching element 17a, 17b open/close pattern setting unit18a, 18b feeding-pulse-energy changing unit 19a, 19b oscillator 20discharge-frequency detecting unit 21 discharge-frequency determiningunit 22 feeding-pattern changing unit 23a, 23b current sensor 24discharge-position detecting unit 25 small-current selecting switchingelement 26 large-current selecting switching element

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a wire electrical discharge machining apparatusaccording to the present invention will be explained below in detailwith reference to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according tothe first embodiment of the present invention. In FIG. 1, referencenumeral 1 denotes a wire electrode. The wire electrode 1 runs, forexample from upward to downward, while being guided by wire guides 2, 3arranged in an up-down direction with an appropriate intervaltherebetween. A plate-shaped workpiece 4 having a certain thickness isarranged on a plane perpendicular to a wire running direction to beopposed to a wire running path between the upper and lower side wireguides 2, 3 with a predetermined machining gap therefrom (hereinafter,“inter-electrode gap”). An upper feeding point 5 is provided at aposition near the upper-side wire guide 2 and a lower feeding point 6 isprovided at a position near the lower-side wire guide 3. The wireelectrode 1 is slidable on the upper and lower feeding points 5, 6.

Machining liquid nozzles, although not shown, are provided on the wirerunning path between the wire guides 2, 3 at positions that are close inthe up-down direction with an opposing position to the workpiece 4therebetween. High-pressure machining liquid is ejected from themachining liquid nozzles into the opposing gap between the wireelectrode 1 and the workpiece 4 from upward and downward, so that thewire electrode 1 can be cooled and discharge machining swarf can beremoved.

A general configuration of a discharge machining unit is explainedabove. The wire electrical discharge machining apparatus includes asub-discharge power supply 7 and a main-discharge power supply 8 asmachining power supplies for the discharge machining unit. Thesub-discharge power supply 7 mainly generates a voltage pulse of arelatively low voltage for supplying a sub-discharge small current tothe inter-electrode gap for detecting a state of the machining gap(inter-electrode gap) between the wire electrode 1 and the workpiece 4.The main-discharge power supply 8 mainly generates a voltage pulse of apredetermined pulse width at a predetermined voltage level that ishigher than the sub-discharge power supply 7 for supplying amain-discharge large current for machining to the inter-electrode gap.Generally, speaking of machining power supply for a wire electricaldischarge machining apparatus, it indicates the main-discharge powersupply 8.

One electrode end of the sub-discharge power supply 7 is directlyconnected to the workpiece 4. The other electrode end of thesub-discharge power supply 7 is connected to the upper feeding point 5through an upper sub-feeder line 9, and an upper sub-switching element10 is inserted on the upper sub-feeder line 9. At the same time, theother electrode end of the sub-discharge power supply 7 is connected tothe lower feeding point 6 through a lower sub-feeder line 11, and alower sub-switching element 12 is inserted on the lower sub-feeder line11.

One electrode end of the main-discharge power supply 8 is directlyconnected to the workpiece 4. The other electrode end of themain-discharge power supply 8 is connected to the upper feeding point 5through an upper main-feeder line 13, and an upper main-switchingelement 14 is inserted on the upper main-feeder line 13. At the sametime, the other electrode end of the main-discharge power supply 8 isconnected to the lower feeding point 6 through a lower main-feeder line15, and a lower main-switching element 16 is inserted on the lowermain-feeder line 15.

Semiconductor switching elements are used here as the switching elements10, 12, 14, 16 and in the following embodiments; however, relays canalso be used in the same manner.

In this manner, there are two circuits in parallel, one on the upperside and the other on the lower side of the workpiece 4, as paths for adischarge current flowing toward the wire electrode 1 from each of thesub-discharge power supply 7 and the main-discharge power supply 8.Moreover, a switching element is provided on each of those current pathsfor opening and closing the path. Therefore, the main discharge currentcan be supplied from the main-discharge power supply 8 while switchingbetween two systems of an upper-and-lower-both-side feeding system usingboth of the upper and lower feeding points and a one-side feeding systemusing either one of the feeding points.

In other words, in the above configuration, when the upper and lowersub-switching elements 10, 12 are simultaneously turned on, the upperand lower sub-feeder lines 9, 11 are closed, and a pulse voltage outputfrom the sub-discharge power supply 7 is applied to the machining gap(inter-electrode gap) between the wire electrode 1 and the workpiece 4through the upper and lower sub-feeder lines 9, 11 and the upper andlower feeding points 5, 6, and a sub discharge (pre-discharge) isgenerated in the inter-electrode gap. In response to this, when the subdischarge (pre-discharge) generated in the inter-electrode gap isdetected, one or both of the upper and lower main-switching elements 14,16 are turned on, and a pulse voltage output from the main-dischargepower supply 8 is applied to the inter-electrode gap to supply the maindischarge current in either one of the one-side feeding system using theupper-side feeding path or the lower-side feeding path and theupper-and-lower-both-side feeding system using both of the upper andlower side feeding paths simultaneously.

Specifically, in the one-side feeding system using the upper-sidefeeding path, when the upper main-switching element 14 is turned on andthe lower main-switching element 16 is turned off, only the uppermain-feeder line 13 is closed, so that the main discharge current issupplied to the inter-electrode gap through the upper main-feeder line13 and the upper feeding point 5.

On the other hand, in the one-side feeding system using the lower-sidefeeding path, when the upper main-switching element 14 is turned off andthe lower main-switching element 16 is turned on, only the lowermain-feeder line 15 is closed, so that the main discharge current issupplied to the inter-electrode gap through the lower main-feeder line15 and the lower feeding point 6.

In the upper-and-lower-both-side feeding system using both of the upperand lower side feeding paths simultaneously, when the upper and lowermain-switching elements 14, 16 are simultaneously turned on, the upperand lower main-feeder lines 13, 15 are closed simultaneously, so thatthe main discharge current is supplied to the inter-electrode gapthrough the upper and lower main-feeder lines 13, 15 and the upper andlower feeding points 5, 6.

The on-off control of the switching elements 10, 12, 14, 16 is performedin response to an output (driving signal) from an oscillator 19 a to bedescribed below. At the time of supplying the main discharge current byperforming the on-off control of the upper and lower main-switchingelements 14, 16, the on-off control of the upper and lower sub-switchingelements 10, 12 is performed in conjunction with the corresponding upperand lower main-switching elements 14, 16 for the purpose thereof. Theon-off control of the upper and lower main-switching elements 14, 16 ismainly explained.

In the first embodiment, in such power supply configuration, anopen/close pattern setting unit 17 a, a feeding-pulse-energy changingunit 18 a, and an oscillator 19 a are provided so that the maindischarge current can be supplied by the main-discharge power supply 8while switching between the upper-and-lower-both-side feeding systemthat concurrently uses upper and lower two feeding points and theone-side feeding system that uses one of the feeding points inaccordance with feeding pulse energy by performing on-off control of theupper and lower main-switching elements 14, 16.

The significance of appropriately switching between theupper-and-lower-both-side feeding system and the one-side feeding systemin accordance with the feeding pulse energy for supplying the maindischarge current by the main-discharge power supply 8 is explainedbefore explaining configuration and operation of the above.

A concentrated discharge can be a cause for wire breakage of the wireelectrical discharge machining apparatus as well as insufficient coolingdescribed in Patent Document 3 (JP 6-61663 B2). If a concentrateddischarge occurs, machining energy on the concentrated dischargeposition of a wire electrode becomes larger than anticipated andoverheating locally occurs, leading to wire breakage. Furthermore, ifthere is deviation of impedance between the upper-side feeding path andthe lower-side feeding path, a difference arises in the dischargecurrent value on the discharge position between feeding from theupper-side feeding path and feeding from the lower-side feeding path.Thus, when performing the upper-and-lower-both-side feeding system only,once the concentrated discharge occurs, the wire electrode is overheatedon a side having a larger discharge current value, causing wire breakageoccurrence more easily.

To prevent occurrence of a concentrated discharge, only one of theupper-side feeding and the lower-side feeding is performed. However,there is still a problem of frequent short circuit occurrences byperforming only one-side feeding, resulting in lowering of the machiningspeed. Accordingly, a measure is considered of mixing theupper-and-lower-both-side feeding with the one-side feeding at anappropriate ratio to prevent frequent short circuit occurrences andenable stable machining.

Specifically, an open/close pattern for opening and closing the upperand lower main-feeder lines 13, 15 is predetermined to mix theupper-and-lower-both-side feeding with the one-side feeding at anappropriate ratio, and power is supplied in accordance with this patternto prevent short circuit. At the same time, if there is a difference indischarge current value between the upper-side feeding path and thelower-side feeding path, the difference can be corrected to prevent wirebreakage.

The problem to be solved when taking these measures is that a differencearises in the machining speeds at the time of the one-side feeding andthe upper-and-lower-both-side feeding as mentioned above. FIG. 2 is adiagram for explaining a difference in impedance between at the time ofupper-and-lower-both-side feeding using upper-and-lower-both-sidefeeding paths and at the time of one-side feeding using one of feedingpaths.

In FIG. 2, (a) corresponds to the discharge current path at the time ofthe upper-and-lower-both-side feeding and (b) corresponds to thedischarge current path at the time of the upper-side feeding. When adischarge point A is in the center in the up-down direction of theworkpiece 4, if the impedances R have same values between the upper andlower main-feeder lines 13, 15 from a feeding-point-side electrode endof the main-discharge power supply 8 to the discharge point A, thedischarge current path from the main-discharge power supply 8 to thedischarge point A at the time of the upper-and-lower-both-side feedinghas two circuits of an upper path and a lower path in parallel, so thatimpedance from the main-discharge power supply 8 to the discharge pointA is R/2. On the other hand, the discharge current path at the time ofthe one-side feeding has only one of the upper-side feeding and thelower-side feeding, so that impedance in the discharge current path isR, which is larger than that at the time of theupper-and-lower-both-side feeding. This results in that the dischargecurrent at the time of the one-side feeding is smaller than that at thetime of the upper-and-lower-both-side feeding. Thus, as the number ofthe one-side feedings increases, the machining speed lowers (see Table1).

TABLE 1 Machining speed ratio when changing ratio of the number ofone-side feedings One-side feedings only 6.3 (One-side feeding ratio100%) One-side feedings + both-side feedings 7.7 (One-side feeding ratio60%) One-side feedings + both-side feedings 8.3 (One-side feeding ratio40%) Both-side feedings only 10 (One-side feeding ratio 0%)

Table 1 shows a verification result of machining speed comparison bychanging the number of power feedings in each power feeding system.Based on the machining speed of 10 when one-side feeding ratio is 0%,that is, when performing upper-and-lower-both-side feedings only, theratio of the number of one-side feedings is changed to 40%, 60%, and100%. The machining speed ratio is 8.3 when the ratio of the number ofone-side feedings is 40%, 7.7 at 60%, and 6.3 at 100%. As the resultshows, the machining speed lowers as the ratio of the number of one-sidefeedings becomes higher.

FIG. 3 is a diagram for explaining current waveforms at power feedingsby various feeding systems shown in FIG. 2. In FIG. 3, (a) is a currentwaveform at the time of the upper-side feeding, (b) is a currentwaveform at the time of the lower-side feeding, and (c) is currentwaveforms at the time of the upper-and-lower-both-side feeding. Thecurrent flowing through the discharge point A shown in FIG. 2 is theupper-side current only at the time of the upper-side feeding as shownin (a), the lower-side current only at the time of the lower-sidefeeding as shown in (b), and the upper-side current and the lower-sidecurrent that are superimposed at the time of theupper-and-lower-both-side feeding as shown in (c), which thereforebecomes larger than the current at the time of the upper-side feeding orat the time of the lower-side feeding.

This means that feeding pulse energy per feeding pulse used at the timeof the one-side feeding can be increased up to nearly same level as thatat the time of the upper-and-lower-both-side feeding that allowsmachining without causing wire breakage. In other words, if taking ameasure for increasing the feeding pulse energy per feeding pulse at thetime of the one-side feeding up to nearly same level as that at the timeof upper-and-lower-both-side feeding, that is, taking a measure forreducing a difference in the feeding pulse energy between at the time ofthe upper-and-lower-both-side feeding and at the time of the one-sidefeeding, the machining speed at the time of the one-side feeding can beimproved.

In FIG. 1, when supplying the main discharge current, the open/closepattern setting unit 17 a performs on-off control of the upper and lowermain switching elements 14, 16 and sets three open/close patterns ofconcurrently closing the upper and lower main discharge current paths,and closing one of the upper and lower main discharge current paths andopening the other path. The three open/close patterns are set such thatthe one-side feeding system and the upper-and-lower-both-side feedingsystem, of which the ratio of the number of power feedings is defined,are mixed at an appropriate ratio to prevent frequent short circuitoccurrences, provide excellent machining speed, and reduce deviation ofthe discharge current value attributable to discharge position. Theopen/close pattern setting unit 17 a outputs the open/close pattern usedfor a present power feeding system, to the feeding-pulse-energy changingunit 18 a.

The feeding-pulse-energy changing unit 18 a recognizes the present powerfeeding system by the open/close pattern from the open/close patternsetting unit 17 a and generates an open/close pattern in which a pulseenergy per feeding pulse is changed in present feeding pulse energy perfeeding pulse that is defined by the open/close pattern received fromthe open/close pattern setting unit 17 a to reduce the difference in thefeeding pulse energy between at the time of the one-side feeding and atthe time of the upper-and-lower-both-side feeding. As can be seen fromthe illustration of FIG. 3 above, the method for reducing the differencein the feeding pulse energy therebetween includes the method of reducingthe feeding pulse energy per feeding pulse at the time of theupper-and-lower-both-side feeding to be smaller than a predeterminedvalue and the method of increasing the feeding pulse energy per feedingpulse at the time of the one-side feeding to be larger than apredetermined value. Which method to adopt is predetermined and isapplied to a recognized present power feeding system.

The oscillator 19 a generates a drive signal for performing on-offcontrol of a corresponding switching element to form a power feedingpath in accordance with the open/close pattern to execute the presentpower feeding system received from the feeding-pulse-energy changingunit 18 a and also generates a drive signal for performing on-offcontrol of a corresponding switching element in the power feeding pathto pour the feeding pulse energy that is changed by thefeeding-pulse-energy changing unit 18 a in the formed power feedingpath.

The method of increasing or decreasing the feeding pulse energy includesthe method of increasing or decreasing a feeding current value and themethod of increasing or decreasing a feeding time length, any of whichcan be employed. When employing the method of increasing or decreasingthe feeding current value, a plurality of the upper and lowermain-switching elements 14, 16 is provided respectively in parallel toenable increasing or decreasing the number of the switching elementsthat are turned on at the same time in each of the upper and lowersides. In this case, the number of the switching elements that areturned on at the same time is determined in accordance with the physicalcharacteristics such as impedance of the power feeding path. Whenemploying the method of increasing or decreasing the feeding timelength, because the upper and lower main-switching elements 14, 16 canbe on-off controlled individually, only the on-operation time length ofeach switching element is increased or decreased. The on-operation timelength to be increased or decreased at this case is also determined inaccordance with the physical characteristics such as impedance of thepower feeding path.

According to the first embodiment, when the power feeding is performedin the above-stated mixed mode of the upper-and-lower-both-side feedingsystem and the one-side feeding system for supplying main dischargecurrent by the main discharge power supply, the difference in thefeeding pulse energy between at the time of theupper-and-lower-both-side feeding and at the time of the one-sidefeeding can be made small, so that the machining speed at the time ofthe one-side feeding using the high impedance path can be improved.

Second Embodiment

FIG. 4 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according tothe second embodiment of the present invention. In FIG. 4, thecomponents that are the same or similar to those shown in FIG. 1 (firstembodiment) are denoted by the same reference numerals. The componentspeculiar to the second embodiment are mainly explained below.

In the second embodiment, as another example for a mixed mode of feedingpower from the high impedance path and power feeding from the lowimpedance path, a case is explained in which one of the upper-sidefeeding path and the lower-side feeding path is configured with a highimpedance path and a low impedance path in parallel.

Namely, as shown in FIG. 4, in the wire electrical discharge machiningapparatus according to the second embodiment having the uppermain-feeder line 13 as an upper-side feeding path and the lowermain-feeder line 15 as a lower-side feeding path in the configurationshown in FIG. 1 (first embodiment), for example, the upper main-feederline 13 is configured with two parallel feeder lines of an uppermain-high-impedance-feeder line 13 a with impedance Z1 and an uppermain-low-impedance-feeder line 13 b with impedance Z2 that is smallerthan the impedance Z1. An upper main-switching element 14 a is providedin the upper main-low-impedance-feeder line 13 b and an uppermain-switching element 14 b is provided in the uppermain-low-impedance-feeder line 13 b.

According to this configuration, in the one-side feeding using theupper-side feeding path, the power feeding can be switched between thepower feeding from the high impedance path by closing the uppermain-high-impedance-feeder line 13 a by turning the upper main-switchingelement 14 a on and the power feeding from the low impedance path byclosing the upper main-low-impedance-feeder line 13 b by turning theupper main-switching element 14 b on.

In the wire electrical discharge machining apparatus, if a peak value ofthe discharge current varies, machining performance such as machiningspeed, surface roughness, and wire electrode consumption vary. In thiscase, for example, as shown in FIG. 4, when one of main-feeder lines isconfigured with two feeder lines having different impedances, it ispossible to switch between the high-impedance feeder line to whichhigh-peak-value discharge current flows and the low-impedance feederline to which low-peak-value discharge current flows. Thus, themachining performance is adjustable.

In this case, when performing the power feeding using the upper-sidefeeding path, the discharge current that passes through the uppermain-high-impedance-feeder line 13 a is smaller than the dischargecurrent that passes through the upper main-low-impedance-feeder line 13b, so that the machining speed lowers.

Accordingly, in the second embodiment, the open/close pattern settingunit 17 a, the feeding-pulse-energy changing unit 18 a, and theoscillator 19 a respectively sets the open/close pattern, changes thefeeding pulse energy per feeding pulse, and performs the on-off controlof the upper main-switching elements 14 a, 14 b based on the same ideaas explained in the first embodiment, whereby difference in the feedingpulse energy per feeding pulse between at the time of the power feedingfrom the upper main-high-impedance-feeder line 13 a and at the time ofthe power feeding from the upper main-low-impedance-feeder line 13 b ismade small.

Specifically, the feeding pulse energy per feeding pulse at the time ofthe power feeding from the upper main-high-impedance-feeder line 13 a ismade larger than a predetermined value. Otherwise, the feeding pulseenergy per feeding pulse at the time of the power feeding from the uppermain-low-impedance-feeder line 13 b is made smaller than a predeterminedvalue, thus enabling to improve the machining speed at the time of thepower feeding from the high impedance line.

As stated above, according to the second embodiment, as another examplefor a mixed mode of feeding power from the high impedance path and powerfeeding from the low impedance path, even when configuring one of theupper-side feeding path and the lower-side feeding path with the highimpedance path and the low impedance path in parallel, the machiningspeed at the time of the power feeding from the high impedance path canbe improved similarly to the first embodiment.

Third Embodiment

FIG. 5 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according tothe third embodiment of the present invention. In FIG. 5, the componentsthat are the same or similar to those shown in FIG. 1 (first embodiment)are denoted by the same reference numerals. The components peculiar tothe third embodiment are mainly explained below.

In the wire electrical discharge machining apparatus, the machiningspeed increases as discharge frequency raises, however the possibilityof wire breakage increases. On the contrary, the possibility of wirebreakage decreases as the discharge frequency lowers, however themachining speed tends to lower. Accordingly, the third embodimentillustrates an example that enables to change a ratio of the number ofpower feedings between at the time of the one-side feeding which is thepower feeding from the high impedance path and at the time of the powerfeeding from the upper-and-lower-both-side feeding which is the powerfeeding from the low impedance path in the configuration shown in FIG. 1in accordance with the magnitude of the discharge frequency.

That is, as shown in FIG. 5, the wire electrical discharge machiningapparatus according to the third embodiment is configured such that thefeeding-pulse-energy changing unit 18 a is omitted from theconfiguration shown in FIG. 1 (first embodiment), output from theopen/close pattern setting unit 17 a is directly given to an oscillator19 b with a changed symbol, and a discharge-frequency detecting unit 20,a discharge-frequency determining unit 21, and a feeding-patternchanging unit 22 are added to directly provide output from thefeeding-pattern changing unit 22 to the oscillator 19 b.

The discharge-frequency detecting unit 20, for example, usesinter-electrode average voltage between the wire electrode 1 and theworkpiece 4 to detect discharge frequency by determining the dischargefrequency as low when the inter-electrode average voltage is high anddetermining the discharge frequency as high when the inter-electrodeaverage voltage is low.

The discharge-frequency determining unit 21 has a preset threshold valuefor determining a magnitude of the discharge frequency and determinesthe magnitude of the present discharge frequency by comparing amagnitude relation between the discharge frequency detected by thedischarge-frequency detecting unit 20 and the threshold value.

When the discharge frequency is determined as high by thedischarge-frequency determining unit 21, the feeding-pattern changingunit 22 generates an open/close pattern in which the ratio of the numberof power feedings for the one-side feeding is increased in accordancewith the ratio of the number of power feedings for the predeterminedhigh discharge frequency and provides to the oscillator 19 b. Bycontrast, when the discharge frequency is determined as low, thefeeding-pattern changing unit 22 generates an open/close pattern inwhich the ratio of the number of power feedings for theupper-and-lower-both-side feeding is increased in accordance with theratio of the number of power feedings for the predetermined lowdischarge frequency and provides to the oscillator 19 b.

In this manner, the feeding-pattern changing unit 22 changes the ratioof the number of power feedings of the open/close pattern for theone-side feeding and the open/close pattern for theupper-and-lower-both-side feeding set by the open/close pattern settingunit 17 a to the open/close pattern for the one-side feeding and theopen/close pattern for the upper-and-lower-both-side feeding thatrespectively have different ratio of the number of power feedings inaccordance with each discharge frequency.

When performing the present power feeding system in accordance with theopen/close pattern received from the open/close pattern setting unit 17a, upon receiving a changed open/close pattern from the feeding-patternchanging unit 22, the oscillator 19 b uses the ratio of the number ofpower feedings designated by the open/close pattern received from thefeeding-pattern changing unit 22 and does not use the ratio of thenumber of power feedings designated by the open/close pattern receivedfrom the open/close pattern setting unit 17 a to output drive signalsfor performing the on-off control of a corresponding switching elementfor the number of the power feedings.

As stated above, according to the third embodiment, the ratio of thenumber of the one-side feedings is high when the discharge frequency ishigh, and the ratio of the number of the upper-and-lower-both-sidefeedings is high when the discharge frequency is low, so that theimprovement of the machining speed and the prevention of the wirebreakage can be realized.

The third embodiment illustrates an example applied to the firstembodiment, which however can be applied to the second embodimentsimilarly.

Fourth Embodiment

FIG. 6 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according tothe fourth embodiment of the present invention. In FIG. 6, thecomponents that are the same or similar to those shown in FIG. 1 (firstembodiment) are denoted by the same reference numerals. The componentspeculiar to the fourth embodiment are mainly explained below.

The method for uniformly increasing the feeding pulse energy at the timeof the one-side feeding is explained in the first embodiment, howeverheat value per unit volume of a wire electrode gets large at the time ofthe one-side feeding, so that the feeding pulse energy that allowsmachining without wire breakage is lower than that at the time of theupper-and-lower-both-side feeding. That is, as shown in FIG. 1, at thetime of the upper-side feeding, the discharge current flows to thedischarge point from the upper position of the wire electrode 1 withwhich the upper feeding point 5 is slidingly in contact, and at the timeof the lower-side feeding, the discharge current flows to the dischargepoint from the lower position of the wire electrode 1 with which thelower feeding point 6 is slidingly in contact. On the other hand, at thetime of the upper-and-lower-both-side feeding, the discharge currentsflow to the discharge point from both the upper and lower positions ofthe wire electrode 1 with which the upper feeding point 5 and the lowerfeeding point 6 are slidingly in contact.

In this manner, the discharge current is divided into the currents fromthe upper and lower positions of the wire electrode 1 at the time of theupper-and-lower-both-side feeding. By contrast, the discharge currentflows only from one of the upper position and the lower position of thewire electrode 1 at the time of the one-side feeding. Thus, the heatvalue per unit volume of the wire electrode 1 is larger at the time ofthe one-side feeding. As a result, when the feeding pulse energy is madelarger at the time of the one-side feeding, the heat value increasesresulting in that wire breakage tends to occur easily.

In the wire electrical discharge machining apparatus, machining liquidnozzles are provided on a wire running path of the wire electrode 1 atpositions that are close in the up-down direction with an opposingposition to the workpiece 4 therebetween. High-pressure machining liquidis ejected from the machining liquid nozzles into the machining gap fromupward and downward between the wire electrode 1 and the workpiece 4, sothat the wire electrode 1 is cooled, which is a cooling measure takenfor preventing overheating of the wire electrode 1 at discharging. Bythis cooling measure, the both-end sides of the wire electrode 1 in themachining gap are more sufficiently cooled than the center portion inthe up-down direction by the machining liquid.

That is, if utilizing the cooling effect equipped in the wire electricaldischarge machining apparatus, when the discharge positions are on theboth-end sides in the machining gap at the time of the one-side feeding,the feeding pulse energy per feeding pulse can be made larger. On thecontrary, when the discharge position is in the center portion in theup-down direction in the machining gap, the feeding pulse energy perfeeding pulse is not made larger.

Accordingly, in the forth embodiment, a case is explained in which thefeeding pulse energy per feeding pulse is not uniformly increased to besupplied at the time of the power feeding from the high impedance path,instead, the magnitude is changed in accordance with the dischargepositions. Namely, in the configuration shown in FIG. 1 (firstembodiment), as shown in FIG. 6, the wire electrical discharge machiningapparatus according to the forth embodiment further includes a currentsensor 23 a that measures current flowing in the upper sub-feeder line9, a current sensor 23 b that measures current flowing in the lowersub-feeder line 11, and a discharge-position detecting unit 24 thatreceives both outputs such that feeding pulse energy, in which outputfrom the discharge-position detecting unit 24 is given to afeeding-pulse-energy changing unit 18 b with a changed symbol, and thefeeding pulse energy per feeding pulse supplied at the time of theone-side feeding can be changed in accordance with the dischargepositions as the above manner.

The discharge-position detecting unit 24 calculates the dischargeposition based on the current values measured by the current sensors 23a, 23 b. The up-down direction of the machining gap is divided intothree and reference points are provided on the respective divisionpoints on the upper and lower positions for determining the dischargeposition. The positional relation between calculated discharge positionand both the reference points on the upper and lower portions is checkedand the discharge position is determined whether it is on the upper endside, on the lower end side, or near the center in the up-down directionin the machining gap.

Specifically, when the calculated discharge position is on the upperside from the upper reference point, the discharge position isdetermined as being located on the upper end side in the machining gap.When being positioned between the upper and lower reference points, thedischarge position is determined as being located near the center in theup-down direction of the machining gap. When being positioned on thelower side from the lower reference point, the discharge position isdetermined as being located on the lower end side in the machining gap.

The feeding-pulse-energy changing unit 18 b uses the open/close patternused for the present power feeding system received from the open/closepattern setting unit 17 a and the discharge position received from thedischarge-position detecting unit 24 to generate an open/close patternhaving the feeding pulse energy per feeding pulse that is made largerthan that set by the open/close pattern setting unit 17 a at the time ofthe one-side feeding and when the discharge position is on the upper endside or on the lower end side in the machining gap, which is output tothe oscillator 19 a. By contrast, when the discharge position is nearthe center in the up-down direction of the machining gap, thefeeding-pulse-energy changing unit 18 b generates an open-close patternin which the feeding pulse energy is not made larger than the aboveincreased one, which is output to the oscillator 19 a.

The method for increasing and decreasing the feeding pulse energy andthe operation of the oscillator 19 a are the same as those explained inthe first embodiment.

As stated above, when discharging is performed on the discharge positionof the upper end side or the lower-end side in the machining gap atwhich cooling by the machining liquid is sufficiently performed, thefeeding pulse energy at the time of the one-side feeding is large, sothat the machining speed can be improved. When discharging is performedon the discharge position near the center in the up-down direction inthe machining gap at which cooling by the machining liquid tends to beinsufficient, the feeding pulse energy can not be as large as the levelat sufficiently cooled positions, so that overheating of the wireelectrode can be suppressed. Therefore, wire breakage due to overheatingof the wire electrode can be prevented, in accordance of which themachining speed at wire-breakage-occurrence limit can be improved,leading to improved machining speed.

FIG. 6 illustrates a case of installing current sensors in the upper andlower sub-feeder lines respectively. However, the current sensors can beinstalled in the upper and lower main-feeder lines.

The forth embodiment illustrates an example applied to the firstembodiment, which however can be applied to the second embodimentsimilarly.

Fifth Embodiment

FIG. 7 is a schematic diagram illustrating a configuration of a relevantportion of a wire electrical discharge machining apparatus according tothe fifth embodiment of the present invention. In FIG. 7, the componentsthat are the same or similar to those shown in FIG. 1 (first embodiment)are denoted by the same reference numeral. The components related to thefifth embodiment are mainly explained.

The first embodiment illustrates a case of increasing a feeding currentvalue and a feeding time length as the method of increasing the feedingpulse energy at the time of the one-side feeding. The fifth embodimentillustrates the method of increasing the feeding pulse energy at thetime of the one-side feeding by using a different machining power supplythat can supply a machining current larger than that at the time of theupper-and-lower-both-side feeding.

That is, as shown in FIG. 7, the wire electrical discharge machiningapparatus according to the fifth embodiment includes a small-currentmain-discharge power supply 8 a and a large-current main-discharge powersupply 8 b instead of the main-discharge power supply 8 in theconfiguration shown in FIG. 1 (first embodiment). The feeding-point-sideelectrode ends of the small-current main-discharge power supply 8 a andthe large-current main-discharge power supply 8 b are respectivelyconnected to the upper and lower main-feeder lines 13, 15 in parallel.The workpiece-side electrode end of the small-current main-dischargepower supply 8 a is connected to the workpiece 4 through a small-currentselecting switching element 25 and the workpiece-side electrode end ofthe large-current main-discharge power supply 8 b is connected to theworkpiece 4 through a large-current selecting switching element 26 inparallel. The feeding-pulse-energy changing unit 18 a is omitted, andthe small-current selecting switching element 25 and the large-currentselecting switching element 26 perform on-off operations by an output(the open/close pattern) from an open/close pattern setting unit 17 bwith a changed symbol. Similarly to the first embodiment, the oscillator19 a performs the on-off controls of switching elements 10, 12, 14, and16 provided in four feeder lines by the output (the open/close pattern)from the open/close pattern setting unit 17 b.

With the above configuration, the upper and lower main-switchingelements 14, 16 are concurrently turned on to concurrently close theupper and lower main-feeder lines 13, 15 and the small-current selectingswitching element 25 is concurrently turned on at the time of theupper-and-lower-both-side feeding. Thus, the machining current from thesmall-current main-discharge power supply 8 a is supplied to theworkpiece 4 through the upper and lower main-feeder lines 13, 15 and theupper and lower feeding points 5, 6.

At the time of the upper-side feeding, the upper main-switching element14 is turned on to close the upper main-feeder line 13 and the lowermain-switching element 16 is concurrently turned off to open the lowermain-feeder line 15. At the same time, the large-current selectingswitching element 26 is turned on and the small-current selectingswitching element 25 is turned off. Accordingly, the large-currentmain-discharge power supply 8 b supplies machining current to theworkpiece 4 through the upper main-feeder line 13 and the upper feedingpoint 5 to increase the feeding pulse energy per feeding pulse at thetime of the one-side feeding.

At the time of the lower-side feeding, the lower main-switching element16 is turned on to close the lower main-feeder line 15 and the uppermain-switching element 14 is concurrently turned off to open the uppermain-feeder line 13. At the same time, the large-current selectingswitching element 26 is turned on and the small-current selectingswitching element 25 is turned off. Accordingly, the large-currentmain-discharge power supply 8 b supplies the machining current to theworkpiece 4 through the lower main-feeder line 15 and the lower feedingpoint 6 to increase the feeding pulse energy per feeding pulse at thetime of the one-side feeding.

As mentioned above, according to the fifth embodiment, asmall-current-supplying machining power supply and alarge-current-supplying machining power supply are provided as mainmachining power supplies to supply machining current to aninter-electrode gap, enabling to supply machining current at the time ofone-side feeding larger than that at the time ofupper-and-lower-both-side feeding, so that feeding pulse energy perfeeding pulse at the time of one-side feeding can be made larger. Thus,the machining speed at the time of the one-side feeding that is thepower feeding from a high impedance path can be improved.

The fifth embodiment illustrates an example applied to the firstembodiment, which however can be applied to the second embodimentsimilarly.

INDUSTRIAL APPLICABILITY

As described above, a wire electrical discharge machining apparatusaccording to the present invention is advantageously used to improvemachining speed at the time of power feeding from a high impedance pathin a mixed use of power feedings from the high impedance path and a lowimpedance path.

1. A wire electrical discharge machining apparatus, when comprising ahigh impedance path and a low impedance path as feeding paths forsupplying a main discharge current from a machining power supply to aninter-electrode gap between a wire electrode and a workpiece, the wireelectrical discharge machining apparatus comprising: a path open/closeunit capable of separately opening and closing each of the highimpedance path and the low impedance path; an open/close pattern settingunit that sets an open/close pattern in which a combination of closingone of the feeding paths and opening another one of the feeding paths isdesignated for switching power feeding between the high impedance pathand the low impedance path; a feeding-pulse-energy changing unit thatgenerates a new open/close pattern in which a pulse energy per feedingpulse is changed in a present feeding path indicated by the open/closepattern by the open/close pattern setting unit such that a differencebetween discharge pulse energy applied to the inter-electrode from themachining power supply when feeding power from the high impedance path,and a discharge pulse energy when feeding power from the low impedancepath is reduced; and a drive unit that controls opening and closing ofthe path open/close unit in accordance with the new open/close patterngenerated by the feeding-pulse-energy changing unit.
 2. The wireelectrical discharge machining apparatus according to claim 1, whereinthe new open/close pattern generated by the feeding-pulse-energychanging unit in which the pulse energy per feeding pulse is changed isan open/close pattern with changed magnitude of the main dischargecurrent.
 3. The wire electrical discharge machining apparatus accordingto claim 1, wherein the new open/close pattern generated by thefeeding-pulse-energy changing unit in which the pulse energy per feedingpulse is changed is an open/close pattern with changed time length ofthe main discharge current.
 4. The wire electrical discharge machiningapparatus according to claim 1, wherein the high impedance path is anupper-side path that runs through an upper-side feeding point providedin slidable contact with the wire electrode on an upper side of theworkpiece or a lower-side path that runs through a lower-side feedingpoint provided in slidable contact with the wire electrode on a lowerside of the workpiece, and the low impedance path is a path that usesboth of the upper-side path and the lower-side path.
 5. The wireelectrical discharge machining apparatus according to claim 1, whereinthe high impedance path and the low impedance path are arranged inparallel having different impedances and are one of the upper-side paththat runs through the upper-side feeding point provided in slidablecontact with the wire electrode on the upper side of the workpiece andthe lower-side path that runs through the lower-side feeding pointprovided in slidable contact with the wire electrode on the lower sideof the workpiece.
 6. The wire electrical discharge machining apparatusaccording to claim 1, wherein the machining power supply comprises amain power supply and a sub power supply, wherein the sub power supplysupplies power to the electrode via a lower sub-feeder line and an uppersub-feeder line and wherein the main power supply supplies power to theelectrode via a lower main-feeder line and an upper main-feeder line,and wherein each of the lower main-feeder line, the lower sub-feederline, the upper main-feeder line, and the upper sub-feeder line, hassame path open/close unit.
 7. The wire electrode discharge machiningapparatus according to claim 6, wherein the same path open/close unit isa switch or a relay.
 8. The wire electrical discharge machiningapparatus according to claim 1, wherein the open/close pattern set bythe open/close pattern setting unit is varied depending on a detectedmeasured current.